The present invention relates to a method for surface treatment of metals, particularly metal-coated steel panels such as zinc-plated, aluminum-plated or tin-plated steel panels.
As the metallic surface treating agent, a chromium-containing surface treating agent such as a chromate system or a phosphate-chromate system has heretofore been used broadly and still in use today. However, in view of the recent trend toward more stringent regulatory control for environmental protection, it is likely that the use of such coating systems will be restricted for fear of the toxicity, particularly carcinogenicity, of chromium. Therefore, development of a rust-preventing agent not containing chromium and yet as effective as the chromating agent in imparting corrosion resistance has been awaited. As disclosed in Japanese Kokai Publication Hei-11-29724, the inventors of the present invention previously developed a nonchromate rust-preventing agent comprising an aqueous resin and, as incorporated therein, a thiocarbonyl group-containing compound, a phosphate ion, and water-dispersible silica. Regrettably, however, this system was found to be deficient in storage stability and somewhat poor in corrosion resistance at thin coating thickness. Meanwhile, with regard to silane coupling agents, an acidic surface treating agent containing two dissimilar silane coupling agents is disclosed in Japanese Kokai Publication Hei-8-73775. However, this system is intended to improve finger-print resistance and overcoat adhesion and is quite deficient in corrosion resistance for use in applications where high corrosion resistance and good processability are required after such metallic surface treatment as in the present invention. Moreover, Japanese Kokai Hei-10-60315 discloses a steel structure surface treating agent, which contains a silane coupling agent having a certain functional group reactive with an aqueous emulsion, but the corrosion resistance required here is only that of a degree satisfying comparatively mild test requirements such as those of wet tests and as far as corrosion resistance is concerned, the system is a far cry from a rust-preventing agent system as provided by the present invention. With the above state of the art by way of background, there has been a standing demand for development of a rust-preventing agent expressing sufficient corrosion resistance and overcoat adhesion at thin coating thickness.
The present invention has for its object to provide a method of treating a metallic surface which is suited for application to metals, particularly zinc-plated steel panels, and despite the absence of chromium therein, is capable of imparting high processability and corrosion resistance to steel panels, and a treated steel panel obtainable by said method.
The method of treating a metallic surface according to the present invention comprises
treating a metal-coated steel panel with a nonchromate metallic surface treating agent containing, in each liter thereof,
(a-1) a silane coupling agent and/or a hydrolytic condensation product thereof in an amount of 0.01 to 100 g/l,
(a-2) water-dispersible silica in a proportion of 0.05 to 100 g/l (solids), and
(a-3) a zirconium compound in an amount of 0.01 to 50 g/l in terms of zirconium ion and/or a titanium compound in an amount of 0.01 to 50 g/l in terms of titanium ion,
drying the treated steel panel and
coating it with an anticorrosion coating agent containing, in each liter of an aqueous resin solution or dispersion,
(b-1) a silane coupling agent and/or a hydrolytic condensation product thereof in an amount of 0.1 to 50 g/l,
(b-2) water-dispersible silica in an amount of 10 to 500 g/l (solids) and
(b-3) at least one phosphorus-containing ion selected from among phosphate ion, phosphite ion and hypophosphite ion in an amount of 0.1 to 10 g/l.
The nonchromate metallic surface treating agent for use in the present invention further contains one or more members selected from among sulfide ion, thiosulfate ion, persulfate ion and a triazinethiol compound in an amount of 0.01 to 100 g/l.
In an alternative mode of practicing the method of treating a metallic surface according to the present invention, said anticorrosion coating agent further contains one or more members selected from among sulfide ion, thiosulfate ion, persulfate ion and a triazinethiol compound in an amount of 0.01 to 100 g/l.
The present invention is further directed to a method of treating a metallic surface
which comprises using an anticorrosion coating agent obtainable by adding said components (b-1) and (b-2) to said aqueous resin solution or dispersion and reacting them at a temperature not less than 50xc2x0 C. and not more than the boiling temperature of the resin composition. This method is most suited for surface treatment of zinc-coated steel panels.
The present invention is further directed to a nonchromate-treated steel panel as obtainable by any of the above methods.
The metallic surface treating agent, which is used in the first place in the present invention, contains a silane coupling agent and/or a hydrolytic condensation product thereof as one of its essential components silane compounds. The hydrolytic condensation product of a silane coupling agent means an oligomer obtainable by hydrolytic polymerization of the silane coupling agent.
The silane coupling agent which can be used as above in the present invention is not particularly restricted but includes the following, among others: vinylmethoxysilane, vinyltrimethoxysilane, vinylethoxysilane, vinyltriethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine, N,Nxe2x80x2-bis[3-(trimethoxysilyl)propyl]ethylenediamine, N-(.-aminoethyl)-.-aminopropylmethyldimethoxysilane, N-(.-aminoethyl)-.-aminopropyltrimethoxysilane, .-aminopropyltrimethoxysilane, .-aminopropyltriethoxysilane, . -glycidoxypropyltrimethoxysilane, .-glycidoxy-propyltriethoxysilane, .-glycidoxypropylmethyl-dimethoxysilane, 2-(3,4-epoxycyclohexyl)ethyl-trimethoxysilane, .-methacryloxypropyltrimethoxysilane, .-methacryloxypropyltriethoxysilane, .-mercaptopropyltrimethoxysilane, .-mercaptopropyl-triethoxysilane and N-[2-(vinylbenzyl-amino)ethyl]-3-aminopropyltrimethoxysilane.
The particularly preferred silane coupling agent includes vinylmethoxysilane, vinylethoxysilane, 3-aminopropyltriethoxysilane, 3-glycidoxypropyl-trimethoxysilane, 3-methacryloxypropyltrimethoxysilane, 3-mercaptopropyltrimethoxysilane, N-(1,3-dimethylbutylidene)-3-(triethoxysilyl)-1-propanamine and N,Nxe2x80x2-bis[3-(trimethoxysilyl)propyl]ethylenediamine. These silane coupling agents can be used each alone or in a suitable combination.
In the present invention, said silane compound is caused to be present in a concentration of 0.01 to 100 g/l, preferably 0.5 to 25 g/l, in [each liter of] the metallic surface treating agent. If the concentration of the silane coupling compound is less than 0.01 g/l, the corrosion resistance- and adhesion-enhancing effect of the nonchromate rust-preventive coating agent will be deficient. If the use of the silane coupling compound exceeds 100 g/l, the corrosion resistance-enhancing effect will not be improved any further but rather a cost disadvantage will result.
The metallic surface treating agent of the present invention contains water-dispersible silica. The water-dispersible silica which can be used in the present invention is not particularly restricted but is preferably spherical silica, chainlike silica or aluminum-modified silica, which is lean in sodium and other impurities and weakly basic. The spherical silica includes colloidal silicas such as xe2x80x9cSnowtexNxe2x80x9d and xe2x80x9cSnowtex UPxe2x80x9d (both manufactured by Nissan Chemical) and fumed silica such as xe2x80x9cAerosilxe2x80x9d (Japan Aerosil); the chain like silica includes silica gel such as xe2x80x9cSnowtex PSxe2x80x9d, (Nissan Chemical); and the aluminum-modified silica includes xe2x80x9cAdeliteAT-20Axe2x80x9d (AsahiDenka), all of which are commercially available.
The water-dispersible silica is caused to be present in a proportion of 0.05 to 100 g/l, preferably 0.5 to 60 g/l, on a solid basis, in each liter of the metallic surface treating agent. If the proportion of water-dispersible silica is less than 0.05 g/l, the corrosion resistance will be insufficient, while the use of silica is in excess of 100 g/l will not be rewarded with any further improvement in corrosion resistance but rather detract from the bath stability of the metallic surface treating agent.
The metallic surface treating agent of the present invention further contains a zirconium compound and/or a titanium compound. The zirconium compound includes ammonium zirconyl carbonate, zirconiumhydrofluoride, ammonium zirconium fluoride, potassium zirconium fluoride, sodium zirconium fluoride, zirconium acetylacetonate, zirconium butoxide-1-butanol solution, zirconium n-propoxide and soon. The titanium compound includes titanium hydrofluoride, ammonium titanium fluoride, potassium titanium oxalate, potassium titanium fluoride, sodium titanium fluoride, titanium isopropoxide, isopropyl titanate, titanium ethoxide, titanium 2-ethyl-1-hexanolate, tetraisopropyl titanate, tetra-n-butyl titanate and so on. These compounds may be used each alone or in a suitable combination.
The above-mentioned zirconium compound and/or titanium compound is caused to be present, in each liter of the metallic surface treating agent of the invention, in a concentration of 0.01 to 50 g/l in terms of zirconium ion or titanium ion. If the concentration of the above compound falls less than 0.01 g/l, corrosion resistance will become insufficient. If it exceeds 50 g/l, no improvement will be realized in overcoat adhesion and, in addition, the bath stability will be rather decreased.
The metallic surface treating agent of the present invention further contains at least one sulfur-containing compound selected from the group consisting of a sulfide, a thiosulfate, a persulfate and a triazinethiol compound, and these compounds contribute to corrosion resistance.
The sulfide mentioned above includes sodium sulfide, ammonium sulfide, manganese sulfide, molybdenum sulfide, iron sulfide and vanadium sulfide, among others.
The thiosulfate includes ammonium thiosulfate, sodium thiosulfate and potassium thiosulfate, among others.
The persulfate includes ammonium persulfate, sodium persulfate and potassium persulfate, among others.
The triazinethiol compound includes 2,4,6-trimercapto-S-triazine, 2-butylamino-4,6-dimercapto-S-triazine, 2,4,6-trimercapto-S-triazine monosodium salt, 2,4,6-trimercapto-S-triazine trisodium salt, 2-anilino-4,6-dimercapto-S-triazine, and 2-anilino-4,6-dimercapto-S-triazine monosodium salt, among others.
These compounds can be used each alone or in a suitable combination.
The concentration of the above sulfur-containing compound(s) in the metallic surface treating agent, per liter of the composition, is 0.01 to 100 g/l in terms of a total amount of sulfide ion, thiosulfate ion, persulfate ion and a triazinethiol compound. If the concentration of said ion(s) is less than 0.01 g/l, the expected corrosion resistance-enhancing effect will not be expressed. On the other hand, if the upper limit of 100 g/l is exceeded, the corrosion resistance-enhancing effect will not be improved any further and rather an economic disadvantage will result.
The metallic surface treating agent according to the present invention may be further supplemented with phosphate ions. Compounds capable of liberating phosphate ions in said metal surface treating agent are used for supplementing the treating agent with phosphate ions. As compounds capable of liberating phosphate ions in said metal surface treating agent, there can be mentioned phosphoric acid; ammonium salts of phosphoric acid, such as triammonium phosphate, diammonium hydrogenphosphate and ammonium dihydrogenphosphate; alkali metal salts of phosphoric acid, such as trisodium phosphate, disodium hydrogenphosphate, sodium dihydrogenphosphate, tripotassium phosphate, etc.; alkaline earth metal salts of phosphoric acid, such as zinc phosphate, magnesium phosphate, etc.; iron phosphate, manganese phosphate, phosphorus molybdate and so on.
The above compounds can be used each alone or in a suitable combination. The addition amount of phosphorus-containing compounds is 0.1 to 10 g/l, preferably 0.25 to 3 g/l, in said metallic surface treating agent. If the amount of phosphorus-containing compounds is less than 0.1 g/l, the corrosion resistance that can be obtained will be insufficient. If the upper limit of 10 g/l is exceeded, the stability of the treating agent tends to deteriorate to an unacceptable level.
The above metallic surface treating agent may further contain other components. As such other components, there can be mentioned tannic acid inclusive of its salt, phytic acid inclusive of its salt, and aqueous resins.
In the method of treating a metallic surface according to the present invention, a metal-coated steel panel, such as a zinc-plated, aluminum-plated and tin-plated steel panel, is first treated with the above metallic surface treating agent. Among metal-coated steel panels, the zinc-coated steel panel is particularly suited to this method. The method of treating a metallic surface may comprise applying said metallic surface treating agent to a substrate metallic surface and drying the coat, or comprise heating such a substrate in advance, applying the metallic surface treating agent of the invention, and allowing the coat to dry by utilizing the residual heat of the substrate.
In both cases, the above drying procedure can be carried out at room temperature to 250xc2x0 C. for 2 seconds to 5 minutes. If the limit of 250xc2x0 C. is exceeded, adhesion and corrosion resistance will be adversely affected. The preferred conditions are 40 to 180xc2x0 C. and 5 seconds to 2 minutes.
In the above method of treating a metallic surface, the amount of deposition of said metallic surface treating agent of the invention is preferably not less than 0.1 mg/m2 as a dry coat thickness. If the dry coat thickness is less than 0.1 mg/m2, the rust-preventive effect will be insufficient. On the other hand, if the dry coat thickness is excessive, it will be uneconomical as an under coat and, in addition, cumbersome procedure-wise. Therefore, the more preferred dry coat thickness is 0.5 to 500 mg/m2, particularly 1 to 250 mg/m2.
In the above method of treating a metallic surface, the mode of use of said metallic surface treating agent of the invention is not particularly restricted. Thus, the routine techniques such as roll coating, shower coating, spray coating, dipping and brush coating can be selectively employed.
In the method of treating a metallic surface according to the present invention, the steel panel subjected to surface treatment with said metallic surface treating agent is further coated with the anticorrosion coating agent and dried. In this anticorrosion coating agent, the following aqueous resin solution or dispersion is employed.
The resin which can be used here includes polyolefin resin, polyurethane resin, acrylic resin, polycarbonate resin, epoxy resin, polyester resin, alkyd resin, phenolic resin, and other thermosetting resins, and these are preferably crosslinkable resins. The above resins may be used in a suitable combination. Particularly preferred is a polyolefin resin, a polyurethane resin or a system comprising those two kinds of resins.
In the above anticorrosion coating agent, the concentration of the resin is 1.0 to 800 g/l, preferably 50 to 400 g/l, on a solid basis. If the formulating amount of the resin exceeds 800 g/l, generally the viscosity is increased to interfere with coating workability. On the other hand, if the amount of the resin is less than 1.0 g/l, a sufficient resin coat may not be obtained so that the corrosion resistance will not be as high as desired.
This anticorrosion coating agent contains a silane compound. The silane compound maybe the same compound as used in the metallic surface treating agent described hereinbefore. The preferred silane compounds are also the same as those already mentioned.
In the anticorrosion coating agent, the concentration of said silane compound is 0.1 to 50 g/l, preferably 0.3 to 20 g/l. If the amount of the silane compound is less than 0.1 g/l, its corrosion resistance-enhancing effect will not be sufficient. If the use of the silane compound is in excess of 50 g/l, the corrosion resistance-enhancing effect will not be improved any further but rather a cost disadvantage will result.
In the anticorrosion coating agent for use in the present invention, water-dispersible silica and a phosphorus-containing ion are formulated in addition to said silane compound to insure sufficient corrosion resistance.
As the water-dispersible silica, all the water-dispersible silicas mentioned for said metallic surface treating agent of the invention can be employed. The concentration of water-dispersible silica in the anticorrosion coating agent is 10 to 500 g/l, preferably 25 to 300 g/l. If the addition amount of water-dispersible silica is less than 10 g/l, the corrosion resistance-enhancing effect will be insufficient. On the other hand, if the limit of 500 g/l is exceeded, the corrosion resistance-enhancing effect will not be improved any further but rather a cost disadvantage will result.
The preferred phosphorus-containing ion includes phosphate ion, phosphite ion and hypophosphite ion. These ions can be supplied by adding compounds capable of liberating the corresponding phosphorus-containing ions in aqueous solution to the anticorrosion coating agent.
The compound capable of liberating phosphate ion in the anticorrosion coating agent includes phosphoric acid; ammonium salts of phosphoric acid such as triammonium phosphate, diammonium hydrogenphosphate, ammonium dihydrogenphosphate, etc.; alkali metal salts of phosphoric acid, such as trisodium phosphate, disodium hydrogenphosphate, sodium dihydrogenphosphate, tripotassium phosphate, etc.; alkaline earth metal salts of phosphoric acid, such as zinc phosphate, magnesium phosphate, etc.; iron phosphate, manganese phosphate and phosphorus molybdate, among others.
The compound capable of liberating phosphite ion includes phosphorous acid, ammonium phosphite, sodium phosphite, potassium phosphite and so on.
The compound capable of liberating hypophosphite ion includes hypophosphorous acid, sodium hypophosphite, ammonium hypophosphite, potassium hypophosphite and so on. These phosphorus-containing ions can be used each alone or in a combination of two or more species in the anticorrosion coating agent according to the present invention.
The concentration of phosphorus-containing ion in the anticorrosion coating agent is 0.1 to 10 g/l, preferably 0.25 to 3 g/l. If the concentration of phosphorus-containing ion is less than 0.1 g/l, the corrosion resistance will be insufficient. If it exceeds 10 g/l, the anticorrosion coating agent will undergo gelation to reduce the storage stability or cause a cost disadvantage because the corrosion resistance-enhancing effect will not be improved any further.
The anticorrosion coating agent of the present invention may be further supplemented with at least one sulfur-containing compound selected from the group consisting of a sulfide, a thiosulfate, a persulfate and a triazinethiol compound to thereby improve corrosion resistance. As the sulfur-containing compound, any of the sulfur-containing compounds mentioned for said metallic surface treating agent can be used.
The concentration amount of said sulfur-containing compound in the anticorrosion coating agent is 0.01 to 100 g/l, preferably 0.25 to 50 g/l, in terms of the amount of sulfide ion, thiosulfate ion, persulfate ion and/or triazine compound. If the concentration of the sulfur-containing compound is less than 0.01 g/l, the corrosion resistance-enhancing effect will be insufficient. On the other hand, if the upper limit of 100 g/l is exceeded, the corrosion resistance-enhancing effect will not be improved any further but rather a cost disadvantage will result.
The anticorrosion coating agent to be used in the present invention may further contain other substances. As such other substances, there can be mentioned a pigment, a surfactant, a solvent and so on.
The pigment that can be used here includes a variety of color pigments inclusive of inorganic pigments, such as titanium dioxide, zinc oxide, zirconium oxide, calcium carbonate, barium sulfate, alumina, kaolin, carbon black, iron oxide, etc., and organic pigments.
In said anticorrosion coating agent, an organic solvent can be formulated for improving the film-forming properties of the resin to thereby yield a more uniform, smooth film. The organic solvent is not particularly restricted as far as it is selected from among the solvents in routine use in coatings, thus including solvents in the alcohol series, ketone series, ester series and ether series, for instance.
In the present invention, said anticorrosion coating agent is coated on a steel panel pretreated with the metallic surface treating agent and dried with a hot air current or, alternatively, said steel panel is heated in advance and the anticorrosion coating agent be then coated on the hot steel panel and allowed to dry by taking advantage of the residual heat of the steel plate.
The heating temperature is 50 to 250xc2x0 C. regardless of which of the above procedures is taken. If the temperature is less than 50xc2x0 C. the evaporation of water will be too slow to provide for a satisfactory film and, hence, a sufficient degree of rust-preventing effect. On the other hand, if 250xc2x0 C. is exceeded, the aqueous resin will undergo pyrolysis so that the salt spray resistance and water resistance will be decreased and the problem of yellowing will also develop. The more preferred heating temperature is 70 to 220xc2x0 C. When the substrate is heated to dry after coating, the drying time is preferably 1 second to 5 minutes.
The coating amount of said anticorrosion coating agent in the practice of the present invention is preferably equivalent to a dry coat thickness of not less than 0.1 .m. If the dry coat thickness is less than 0.1 .m, the rust-preventing effect of the product steel panel will be insufficient. On the other hand, coating to an excessively large dry coat thickness is too expensive for an under coat and inconvenience is felt in coating. Therefore, the more recommendable is 0.1 to 20 .m, with the range of 0.1 to 10 .m being particularly preferred.
The coating technology for said anticorrosion coating agent of the invention is not particularly restricted but includes roll coating, air-spray coating, airless-spray coating and dip coating, among others.
Production of said anticorrosion coating agent according to the present invention can be carried out typically in the following manner. A reaction vessel is charged with the starting materials, i.e. said aqueous resin composition and water-dispersible silica, and the mixture is heated to a temperature not less than 50xc2x0 C., preferably not less than 60 xc2x0C., but not more than the boiling temperature of the resin composition while it is constantly stirred. Then, a predetermined amount of said silane coupling agent is added dropwise over 1 to 8 hours for reaction while the system is stirred at the same temperature. On completion of the reaction, the product is cooled and said phosphorus-containing compound is added. Optionally, said sulfur-containing compound is further added and, where necessary, the mixture is adjusted with water or a solvent to a predetermined solid content, whereby the anticorrosion coating agent is obtained.
In the above production method, if there action temperature is less than 50xc2x0 C., the reaction between the silane compound and the aqueous resin and/or water-dispersible silica will not proceed far enough so that the effect of the invention may not be obtained. On the other hand, if the reaction temperature reaches or exceeds the boiling temperature of the resin composition, the evaporation of water will become undesirably violent. The reaction time need be 1 to 8 hours, and a predetermined amount of said silane compound is added within this time period at a rate of 0.1 g/min to 10 g/min. After completion of dropwise addition, the reaction is further continued for about 2 hours. Usually the reaction conditions may be 3 to 5 hours and 80xc2x0 C.
The method of treating a metallic surface according to the present invention comprises treating a metal-coated steel panel with a metallic surface treating agent containing a silane coupling agent and/or a hydrolytic condensation product thereof, water-dispersible silica, and a zirconium compound and/or a titanium compound and, then, coating it with an anticorrosion coating agent containing a resin, water-dispersible silica and a phosphorus-containing ion. Thus, the present invention provides a method of treating a metallic surface which does not employ chromium and yet is capable of imparting excellent processability and corrosion resistance to a PCM steel plate and a steel plate as obtainable by said method.
In accordance with the present invention, an outstanding rust-preventive effect can be achieved by using, in combination, a resin-free metallic surface treating agent as a primary rust-preventing agent and a resin-containing anticorrosion coating agent as a secondary rust-preventing agent. The suspected mechanism is that as an aqueous solution containing a rust-preventive component alone is applied before application of a resin-containing anticorrosion coating agent, said primary rust-preventing agent is firmly attached in a sufficient amount to the base metal surface to thereby phenomenally enhance the adhesion between the aqueous resin to be applied subsequently and the metal substrate. As a result, even if injuries or other film defects develop in the anticorrosive layer, the firm bond between the rust-preventing agent and the metal surface enables rapid passivation of the metal surface defects by the ionization due to penetrating water to thereby bring about a marked improvement in corrosion resistance. An effect like this can never be provided by a one-coat layer formed with either a metal surface treating agent or a rust-preventing agent-containing resin system.
The steel panel obtainable by the method of the invention has excellent processability and corrosion resistance and, therefore, can be used broadly in such fields as household electrical appliances, computer-related equipment, architectural members, and automotive and other industrial products.